Systems and methods for monitoring heart function

ABSTRACT

Characteristics of a user&#39;s heart are detected. In accordance with an example embodiment, a ballistocardiogram (BCG) sensor is used to detect heart characteristics of a user, and provide a BCG output indicative of the detected heart characteristics. The BCG output is further processed using data from one or more additional sensors, such as to reduce noise and/or otherwise process the BCG signal to characterize the user&#39;s heart function.

RELATED PATENT DOCUMENTS

This patent document claims the benefit 35 U.S.C. §119(e) of U.S.Provisional Patent Application No. 61/105,696, filed on Oct. 15, 2008,which included five appendices (A through E) that provide example andexperimental results for use with various embodiments of the presentinvention, and which further cite various references, all of which arefully incorporated herein by reference.

FIELD OF INVENTION

This invention relates generally to monitoring of heart function, and inspecific instances to systems and methods for detecting heart functionand processing signals therefor.

BACKGROUND

The National Institutes of Health (NIH) estimate that over 500,000annual hospital admissions for heart failure patients are avoidable.Heart failure is a tremendous problem in the United States that affectsmillions of people and costs billions of health care dollars each year.A number of tests allow a doctor to assess heart function; however,proper implementation of many of these tests requires expensiveequipment and/or a qualified professional. This makes it difficult andexpensive to monitor the condition of a patient, as the patient mustroutinely visit the hospital or risk late detection of a worsening heartcondition.

These troubling numbers have led researchers and companies to improvethe methods for monitoring heart failure outpatients with the goal ofreducing unnecessary hospital readmissions and costs. One proposedsolution transmits a patient's body weight measured on a bathroom scaleto his/her physician because increasing body weight could be a sign offluid retention, which might result from a worsening heart condition.Such diagnostic mechanisms are, however, less than ideal. Body weightcan be affected by factors other than fluid retention, and fluidretention can be caused by factors other than heart conditions. Thus,such techniques result in false positives, or worse, fail to detectworsening heart conditions.

The ballistocardiogram (BCG) is a non-invasive diagnostic techniquewhich measures the mechanical recoil of the body in reaction to theejection of blood from the heart through the vasculature. BCGmeasurement methods were developed in the mid-1900s and largelyabandoned in the early 1960s due to impracticality and unreliability.Reliable and practical BCG signal acquisition would provide an excellenttool for evaluating a patient's cardiovascular health. However, suchsignal acquisition remains challenging for use in a variety ofapplications.

SUMMARY

The present invention is directed to approaches to systems and methodsfor monitoring of heart function. The present invention is exemplifiedin a number of implementations and applications including thosepresented below, which are commensurate with certain claims includedwith this patent document.

According to an example embodiment, a system provides BCG(ballistocardiogram) data from a user. The system includes a BCG capturedevice, a secondary sensor and a processor circuit. The BCG capturedevice includes a heart-characteristic sensor that captures, from theuser, a BCG signal indicative of at least one of physical movement andmechanical output of the user's heart. The secondary sensor detects anindication of at least one of noise present in the BCG signal and aphysiologic characteristic of the user, and provides an outputcharacterizing the detected indication. The processor circuit uses thesecondary sensor output to process the captured BCG signal and togenerate an output BCG signal indicative of a condition of the user'sheart. For example, the BCG signal may be processed via filtering oranother type of modification that uses the secondary sensor's detectionas an input to alter the signal.

Consistent with another example embodiment of the present invention, asystem/method provides ballistocardiogram (BCG) measurements (e.g., inreal-time) from a user standing on a BCG capture device. A force sensoris arranged to capture a signal indicative of the physical movementand/or mechanical output of the heart of the user while the user isstanding on the device. A second specific sensor type (e.g., ECG,accelerometer, geophone, displacement, electromyogram or video imagingdevice) provides additional information about the captured signal, whichmay be indicative of noise and/or interference present in the BCGmeasurement, or of other characteristics of the user. A processor usesthe second-sensor signal to process the captured signal, such as tofilter or gate (e.g., weight or eliminate aspects of) a captured BCGrecording, and provide user diagnostics.

In some implementations, a captured BCG recording is gated to weight oreliminate segments of the recording that contain higher than usablenoise or interference levels (e.g., for averaging algorithms) Forexample, regions of higher noise can be given proportionally lowerweighting in weighted ensemble averaging algorithms, such as maximumlikelihood averaging.

Another example embodiment is directed to a system for providing BCG(ballistocardiogram) data from a user. The system includes a BCG capturedevice, a secondary sensor and a processor circuit. The BCG capturedevice includes a heart-characteristic sensor that captures, from theuser, a BCG signal indicative of at least one of physical movement andmechanical output of the user's heart. The secondary sensor detects anindication of at least one of noise present in the BCG signal and aphysiologic characteristic of the user (e.g., a heart characteristicand/or a noise-based characteristic), and provides an outputcharacterizing the detected indication. The processor circuit uses thesecondary sensor output to process the captured BCG signal and generatean output BCG signal indicative of a condition of the user's heart.

The above summary of the present invention is not intended to describeeach illustrated embodiment or every implementation of the presentinvention.

BRIEF DESCRIPTION OF THE FIGURES

The invention may be more completely understood in consideration of thedetailed description of various embodiments of the invention thatfollows in connection with the accompanying drawings, in which:

FIG. 1A shows a block diagram of a system and approach for determiningcardiac function, consistent with an example embodiment of the presentinvention;

FIG. 1B shows a block diagram of a system and approach for detectingcardiac function, consistent with another example embodiment of thepresent invention;

FIG. 2 shows a circuit diagram for simultaneously exciting the forcesensor bridge in orthogonal configurations with two differentfrequencies, consistent with another example embodiment of the presentinvention;

FIG. 3 shows a block diagram of a system and approach for detectingcardiac function and mitigating the effects of vibration in the localenvironment, according to another example embodiment of the presentinvention; and

FIG. 4 shows a circuit for acquiring BCG signals from a commercialweighing scale, consistent with another example embodiment of thepresent invention.

While the invention is amenable to various modifications and alternativeforms, examples thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments shown and/or described. On the contrary, theintention is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention.

DETAILED DESCRIPTION

Various embodiments of the present invention have been found to beparticularly useful in connection with monitoring heart function (e.g.,to determine cardiac health of a patient) in a manner that facilitateshome use by the patient. While the present invention is not necessarilylimited to such applications, various aspects of the invention may beappreciated through a discussion of various examples using this context.

Aspects of the present invention are directed to detecting the heartfunction of a user with a sensor that detects weight and/or weightvariances of a user. A processing arrangement is configured (e.g., withan algorithm/transform) to determine heart function characteristics ofthe user based upon the detected weight and/or weight variances. Theprocessing arrangement uses data from one or more additional sensors asa parameter of the algorithm/transform. In connection with these exampleaspects, it has been discovered that such implementations can beparticularly useful for producing unexpectedly practical and reliableBCG measurements.

In another example embodiment, a BCG (ballistocardiogram) systemincludes a BCG capture device including a heart-characteristic sensorcaptures, from a user, a BCG signal indicative of at least one ofphysical movement and mechanical output of the user's heart. A secondarysensor detects a secondary characteristic relating to the BCG signal,and provides an output characterizing the detected indication. Forexample, the secondary sensor may detect characteristics of a userand/or of the user's environment to provide an indication of one or moreof noise present in the BCG signal and a physiologic characteristic ofthe user. A processor circuit uses the secondary sensor output toprocess the captured BCG signal and to generate an output BCG signalindicative of a condition of the user's heart.

In some implementations, the BCG capture device includes a weighingscale, and the secondary sensor includes an electrocardiogram (ECG)sensor that detects an ECG signal from the user, or aphotoplethysmograph sensor that detects blood flow pulsations of theuser. This detected signal is used to process a signal obtained via theBCG capture device.

In another implementation, the secondary sensor includes an ECG sensorthat detects an ECG signal from the user that is indicative of, orotherwise useful for determining, characteristics of the user andrelated BCG signal capture. The processor circuit uses an algorithm toprocess the captured BCG signal and to generate the output BCG signalusing the detected ECG signal as an input to the algorithm to processthe BCG signal. In certain applications, the processor circuit generatesan output BCG signal based upon an ensemble-average of the detected BCGsignal generated via the detected ECG signal. This averaging can be bothstatic—providing a single ensemble-averaged BCG beat—or dynamic, as insynchronous moving averaging or exponentially-weighted triggeredaveraging.

In some implementations, data is detected for both BCG- and ECG-basedanalysis using a strip-type sensor or a handlebar-type sensor that maybe implemented on a scale device as discussed herein (see, e.g., FIG. 1Aand FIG. 1B, discussed further below). One or more such sensors are usedto effectively capture a signal from a user that is subsequentlyprocessed to generate both BCG and ECG analysis data. In someimplementations, ECG data that is detected and/or generated is used inthe generation of BCG analysis data, such as by filtering a capturedsignal to facilitate the representation of one or more of physicalmovement and mechanical output of the user's heart. For example, BCG andECG (or photoplethysmograph) signals can be adaptively filtered, orprocessed via ECG R-wave (or photoplethysmograph peak) triggeredensemble averaging or triggered moving averaging, to improve thesignal-to-noise ratio and the consistency of BCG recordings.

In some embodiments, a strip-type or handlebar-type sensor as discussedabove includes a two-electrode ECG circuit configured for contacting thehands of a user. In some implementations, the two-electrode ECG circuituses active current feedback to one electrode to reduce amplifiersaturation problems, rendering higher signal quality on the ECGrecordings. Detected ECG characteristics can then be used for adaptivelyfiltering, ensemble averaging, or otherwise processing the BCG signalmeasured from the force sensor in the scale, to improve the signalquality of the BCG.

In some implementations, the ECG or photoplethysmograph (or otherreference signal) is adaptively filtered to estimate the BCG to mitigaterequirements or needs for detecting peaks or heartbeat segmentation, oruse of an R-wave detector (e.g., for ensemble averaging or triggeredmoving averaging). In some implementations, an ECG orphotoplethysmograph signal is input directly into an adaptive filter,with the raw BCG signal as the desired response; the output of thisfilter, to form a best least-squares estimate of the signal without anyneed for ECG or photoplethysmograph peak detection. In someimplementations, a least-mean squares algorithm is used to adapt weightsof the adaptive filter. The convergence rate can be chosen to be slow,allowing the filter to converge to the best solution for the user of thedevice.

Another embodiment is directed to a BCG capture device as discussedabove, including a weighing scale that includes the heart-characteristicsensor. The processor circuit processes the captured BCG signal, usingpreviously-obtained data including cardiac output and stroke volume datafor the user (e.g., derived from Doppler ultrasound measurements) andboth rms power and an ensemble-average amplitude of the BCG signal, togenerate an estimated output BCG signal indicative of the cardiac outputand stroke volume of the user's heart. In some implementations, theprocessor circuit receives the previously-obtained cardiac data from aremote location. In other implementations, the processor compares thecaptured BCG signal to a baseline value, which is based upon cardiacoutput and stroke volume data for the user and both rms power and anensemble-average amplitude of the BCG signal, to generate an output BCGsignal indicative of the cardiac output and stroke volume of the user.

A specific implementation involves the use of a scale having customelectronic circuitry for acquiring and processing the BCG signal. Usersof the scale position themselves on the scale. The weight is measuredand recorded as a function of time. The sensitivity of the measurementis sufficient in both weight and sample speed so that thegenerated/recorded signal contains the desired BCG signal. Forrelatively small BCG signals (compared to a number of other sources offorce variances such as respiration, user movement, building vibrationsand/or electrical noise), aspects of the present invention are directedto detecting the BCG signal, relative to one or more of the aforesaidsources. In some implementations, the amplitude of the BCG signal iscorrected based upon the weight of a user, based on kinetic energytransfer. A variety of scales, customized or otherwise coupled foracquiring a BCG signal, can be implemented in connection with variousexample embodiments. For instance, the Omron HBF-500 Body CompositionMonitor/Scale available from Omron Healthcare Inc., of Bannockburn, Ill.can be implemented in connection with one or more example embodiments.

In another example embodiment, a BCG characteristic is detected inconnection with the detection of characteristics in an implantablepacemaker or cardioverter defibrillator (ICD), and can further be usedin connection with the characteristics of the pacemaker or ICD foranalysis and/or correction. For instance, the detected BCGcharacteristic can indicate the responsiveness of the cardiovascularsystem to various settings within the pacemaker, and can be used forimproving the responsiveness or coordination of heart contractions byvarying the parameters of the implantable device. In someimplementations, the baseline heart rate setting of a pacemaker can beincreased or decreased based on changes over time in a person's BCGcharacteristic, such as a decreasing trend in BCG rms power over severaldays or weeks. In some implementations, the pacing pulse timings of thepacemaker can be used for triggered averaging, moving averaging, oradaptively filtering the BCG signal of the user. Since the timing ofthese pacing pulses is deterministically known by the processor, theaveraging or filtering algorithms will be more accurate than, forexample, if the R-wave peak of the ECG characteristic was used foraveraging or filtering.

In another example embodiment, a BCG characteristic is detected inconnection with the detection of characteristics for pacemakeradjustment, such as in cardiac resynchronization therapy (CRT), and canfurther be used in connection with CRT-based analysis and/or correction.For instance, the detected BCG characteristic can be used in suppressingundesirable heart functions and/or in otherwise coordinating heartcontractions. In connection with these embodiments, it has beendiscovered that BCG characteristics can be detected while pacemakerparameters are adjusted as part of CRT-based control, and used toidentify desirable pacemaker parameters via corresponding BCGcharacteristics. For example, a statistically significant increase inmaximum BCG amplitude can be obtained for various (e.g., six) subjectsat desirable or optimal pacing settings, versus other/suboptimalsettings (p<0.05).

In one implementation, a processor circuit of a BCG system as describedabove automatically sets cardiac resynchronization therapy (CRT) paceparameters for the user (e.g., as a maximum amplitude or minimum ECGR-wave to BCG J-wave interval) based on the output BCG signal. In someimplementations, the BCG system further includes an implantable devicehaving an accelerometer, and the processor circuit sets the CRT paceparameters based upon an output from the accelerometer. Theaccelerometer can be integrated within an implantable pacemaker, andcoupled with lead-derived ECG, can be used with one or more of thetechniques discussed herein as a BCG analog for automaticself-adjustment (e.g., when the patient is standing on somethingsomewhat compliant) to provide feedback regarding cardiac output to thepacemaker.

Other implementations are directed to using a BCG system as discussedabove to generate a visual or other indication that can be used to setpacemaker parameters. The processor circuit generates an output BCGsignal indicative of BCG signal variations due to changes in pacemakerparameters. For CRT, these parameters could include, for example, theinterventricular pacing delays, or the atrio-ventricular pacing delays.

These CRT-based implementations are also amenable for open-loop orclosed-loop adjustment of implantable pacemakers in a clinic setting.For example, a doctor can adjust the pacemaker settings with specifichemodynamic feedback. Such settings may involve, for example, adjustinga timing delay between pacing in different chambers to achieve a featureon a detected BCG signal corresponding to maximum cardiac output ormaximum cardiac contractility. In certain applications, a wireless linkis used for wireless communication between the implanted pacemaker and ascale (or other device) detecting BCG characteristics while a subjectstands on the scale to provide for closed-loop pacing parameteradjustment.

While adjusting pacemaker settings, BCG measurements are monitored andused to detect ejection force and/or other heart-based characteristics.Using this approach, desirable pacemaker settings can be identified asthose that result in a desirable ejection force. This monitoring andrelated settings can be carried out visually, such as by providing avisual signal indicative of a patient heart's ejection force, orautomatically using a circuit or programmed computer to automaticallygenerate a desirable setting in response to detected BCGcharacteristics.

In some implementations, the pacing pulses are used as triggers forensemble averaging, triggered-averaging, or otherwise filtering measuredBCG signals. With this approach, it has been discovered that a higherquality averaged or filtered BCG signal can be obtained than by usingthe timing of the ECG QRS complex itself. The pacing pulse timings canbe extracted, for example, from the processor in the implantable device,the pacemaker programmer device in the clinic, or simply from pacingpulse artifacts in a simultaneously-measured surface ECG signal.

Another example embodiment is directed to a BCG system as describedabove, having a bodyweight sensing scale with an ECG and/orphotoplethysmograph sensor integrated into the scale platform,detachable unit, or separate unit connected to the scale. In someimplementations, the BCG capture device is integrated with thebodyweight sensing scale, and the secondary sensor is integrated withhandlebar electrodes. The electrodes and secondary sensor detect atleast one of an electrocardiogram (ECG) or photoplethysmographycharacteristic of the user. The processor circuit generates output BCGsignals over time to provide an indication of at least one of cardiacoutput and stroke volume for determining a treatment need for the user,such as for titration of care for the user (e.g., for the adjustment ofmedicine dosage (with physician consultation) or signaling the need fora clinical visit).

In some implementations, the BCG capture device is integrated with thebodyweight sensing scale, and the secondary sensor is integrated in atleast one of the scale platform, a detachable module, or a separatemodule connected to the scale via hardwire or wireless link. Thesecondary sensor detects a photoplethysmography characteristic of theuser. The processor circuit generates output BCG signals over time toprovide an indication of at least one of cardiac output and strokevolume for titration of care for the user.

In some implementations, changes in the BCG signal over time can becoupled with changes in bodyweight over time to assess a patient'soverall health. For example, for heart failure patients, the fluidstatus indication derived from bodyweight changes can be supplemented bythe cardiac output or stroke volume changes estimated by the BCG sensorto assess both congestion and perfusion of the patient. For instance,increasing body weight coupled with decreasing BCG rms power couldindicate worsening condition and the need for increasing diuretic dosage(with physician consultation) or a clinical visit.

Another example embodiment is directed to screening patients such asathletes to generate data from BCG signals. A processor circuit asdescribed above is configured to generate an output BCG signalindicative of BCG rms power to provide an indication of cardiac fitness.This approach can be used, for example, to characterize ventricular massof a patient being screened to detect abnormalities or otherheart-related conditions, in addition to and/or as an alternative todetecting other heart characteristics as described herein. In connectionwith these embodiments, it has been discovered that detected BCGcharacteristics can be indicative of left ventricular mass (LVM) of thepatient being screened. A detected BCG signal is integrated, and a highor maximum value of the integrated signal is used as an indicator ofmomentum change. In many implementations, a patient's weight is alsoused in connection with a detected BCG signal to characterize LVM.

In one implementation, a scale is modified for detecting BCG andsingle-lead ECG (e.g., as shown in FIG. 1B and described below). BCG andECG signals are amplified by and interfaced to a computer for recording.The BCG beats for each subject are averaged using R-waves of the ECG asa trigger. In such implementations, a correlation between maximummagnitude of the BCG integral (MMBI) and LVM is significant (e.g.,R=0.68, p<0.01), and the correlation between the product of MMBI andbody weight, and LVM is stronger (e.g., R=0.75, p<0.01).

Other athletic characteristics, such as exercise recoverycharacteristics, can also be detected using these approaches.Measurement, processing, and analysis of all signals during exerciseand/or recovery can be useful in evaluating an athletic condition. Thisevaluation can be particularly useful when compared to evaluating heartrate alone, as BCG characteristics as discussed herein can be correlatedto work exerted by a user's heart per stroke, which can be used toevaluate exercise recovery in a manner that goes beyond the evaluationof a user's heart rate. For example, one implementation is directed tousing a detected BCG characteristic to evaluate exercise recovery basedon the time it takes for various parameters to return to their basalvalues or the maximum exertion achieved by the heart.

Turning now to the figures, FIG. 1A shows a block diagram of a systemand approach for determining cardiac function/characteristics of apatient, consistent with an example embodiment of the present invention.User 102 positions themselves on scale 100. Scale 100 includes aforce/weight sensor 106. This sensor 106 is configured to detect weightvariations that are converted to an electrical signal that is sent toprocessing arrangement 108, and can be further used to provide theweight of the user, such as that provided by a traditional scale.

Secondary input(s) 104 provide information in addition to strict weightsensing. These inputs can include, for example, signals provided byheart-beat sensors, foot-to-foot impedance cardiogram sensors, usermovement sensors and the like, which may further be incorporated intothe system. In some implementations, impedance plethysmography orphoto-plethysmography sensors, can be used to improve signal processing.

Still other implementations are directed to addressing motion artifacts,such as by using a secondary non-contact displacement sensor to measurecorrelated cardiac related information and uncorrelated body motionnoise and remove motion artifacts. One such implementation involvesusing a displacement transducer such as an optical or acousticalemitter/detector sensor, to measure absolute or relative changes in bodymotion to improve the cardiac signal. Another approach to removingmotion artifacts involves using multichannel sampling on individualstrain gages (e.g., as described in connection with FIG. 2), or byswitching a bridge circuit to capture different motion axes.

The processing arrangement 108 is configured to use the secondaryinput(s) 104 and various filtering algorithms to remove extraneous noiseor interference on the signal from sensor 106. The results of thisprocessing/filtering can be sent to an output 110, such as an LCDdisplay or local memory. This information can be presented in arecordable form, such as for recording by the patient using the system,or for uploading for access by a doctor at a remote location. In someinstances, the output 110 includes a network interface type device thatprovides an output to a network (e.g., Ethernet or wireless) connectedto one or more storage devices that receive output data from scale 100.In other instances, the output 110 includes one or more of a UniversalSerial Bus (USB) connection, removable memory card device, contactlesscard device, or a relatively simple indictor that indicates thatabnormal cardiac function has been detected (e.g., warning the patientto contact a doctor).

According to one implementation, an ECG signal (single or multiple lead)is recorded simultaneously with weight-related detection (e.g., weightvariations as discussed above) and used as a secondary input conditiontogether with the detected weight-related condition. The signals fromthese recordings are combined using adaptive filtering, such as byadaptively filtering a secondary signal to determine the bestleast-squares estimate of the BCG signal from a raw weight measurementwaveform. This approach leverages the fact that the ECG and BCG signalsare correlated in time, while the noises in these waveforms arestatistically independent.

In another implementation, heart rate (e.g., ECG orphotoplethysmogram)-triggered ensemble averaging is used to enhance thequality and consistency of the BCG signal. Such a technique leveragesoff the ability to easily detect heart contraction using any of a numberof different techniques. Detected heart contractions are then used toselect relevant portions of the detected weight measurement to use inensemble averaging. For example, ensemble averaging may be used inconnection with detected noise to mitigate noise in the BCG signal.

In various implementations, an adaptive filtering approach using a leastmean squares algorithm is used to remove noise from the BCG signal withthe ECG signal as a reference. BCG signals are ensemble averaged usingthe ECG R-wave as the trigger. This ensemble averaging can be bothstatic (one average for the entire data set) and dynamic (synchronousmoving average). Additionally, respiration signal can be used as areference for adaptive noise cancellation of respiration from the BCG.In certain applications, the system in FIG. 1A is configured forself-calibration to eliminate instrumentation resonance, to decouplemechanical filtering of the signal by the scale other device used tocapture the BCG signal.

In another embodiment, force-balanced BCG is used to reduce or eliminatethe need to mechanically dampen the device 100 supporting the user. Thisapproach is applicable to other sensor-type arrangements, such as asensor located in a prone-position scale, a chair, a bed or otherwise.

FIG. 1B shows a block diagram of a system 120 for detecting cardiacfunction involving both ECG and BCG detection, consistent with anotherexample embodiment of the present invention. The system 120 can be usedin a manner similar to that as described above in connection with FIG.1A, and further with the above examples using both ECG and BCG, fordetecting conditions of a user's heart. A scale-type BCG sensor device130 includes an ECG-type hand-held sensor 132 that detects ECGcharacteristics that are used in connection with BCG characteristicsdetected at the scale-type device 130. An output from the BCG sensordevice 130 is passed to a processor 140 that processes the output anddetected ECG and BCG characteristics therein, to determine a heart-basedcondition of a user 122. In various implementations, one or moreadditional sensors, represented at block 150, are also coupled to theprocessor 140, which uses the inputs as part of the determination of theheart-based condition (e.g., such as a vibration sensor that is used toremove noise in one or both of a BCG- and ECG-based signals).

In some applications, e.g., where the use of a secondary sensor may notbe practical or is otherwise undesirable, a force sensor is arrangedsuch that two force signals can be measured with correlated cardiacrelated information and incoherent motion noise. FIG. 2 shows an exampleembodiment amenable to implementation in such applications. A sine-wavevoltage source 200 is configured to operate at a frequency that ishigher than the frequency of the heart rate (e.g., 1 kHz), and iscoupled to inputs A and C of a strain gauge circuit 212 via a firstoutput winding of a transformer 201 (T1). The strain gauge circuit 212is configured to vary the resistance of the bridge by a factor of alpha213, which is proportional to the force imparted on the strain gauges.

The BCG signal, modulated by the frequency of the sine-wave source 200,appears across the terminals B and D and is AC-coupled by capacitor 209into a synchronous demodulator and gain block 210. The second outputwinding of transformer 201 provides the synchronization signal to thedemodulator and gain block 210. The output of 210 is the first BCGsignal 211. A DC voltage 207 is input to resistor R1 208. This causes aBCG signal to appear across terminals A and C of the strain gaugecircuit 212, which is passed through transformer 202 (T2) to a filteringcapacitor 203 and a difference amplifier 204. Using the characteristicsof the AC signal produced by the sine-wave source 200 (which is at afrequency much higher than the heart rate), an RC low-pass filter 205 isused to attenuate this sine-wave signal, leaving the second BCG signaloutput 206.

The specific circuit depicted by FIG. 2 is exemplary of a number ofdifferent implementations that can be used to provide similarfunctionality. As with other aspects of the present invention, thevarious functionalities can be implemented using combinations of generalpurpose computers configured by specialized software, programmable logicdevices, discrete circuits/logic and combinations thereof.

In connection with various embodiments, acquired signals as describedherein are used in deriving/monitoring various different types ofinformation including, but not limited to, heart rate, the force ofejection of blood from heart (which can be correlated to cardiacoutput), time delay from electrical depolarization to mechanicalcontraction of the ventricles, relationship between electrical andmechanical activity in the heart (relating to excitation-contractioncoupling), diagnosis of various diseases (including congestive heartfailure, coarctation, aneurysm, etc.), predicting future cardiac healthtrends and/or non-invasive blood flow and pressure measurements.

Various aspects of the present invention are directed to use in a homeor other location where it may not be practical to have a trainedtechnician or physician available. In one implementation, simultaneousBCG and ECG recordings from a commercial bathroom scale or chair areused to facilitate home monitoring of cardiovascular health in a compactand inexpensive platform for reliable BCG acquisition. BCG measurementscan be implemented for chronic management of congestive heart failure(CHF) patients at home. Various methods and devices discussed herein canbe useful for tracking fluid loading by both body weight and relativecardiac output. For instance, an increased weight measurement anddecreased BCG signal may be used as an indication of worsening heartfailure.

Certain embodiments are directed to using blood flow measurementsdetected via ultrasound to calibrate BCG features for each person. Thiscan be particularly useful as each person's BCG may be different due todifferent physiology. A blood flow waveform can be used from ultrasoundto precisely determine transfer function relating vascular flow to BCGsignal. This transfer function could then be inverted and applied to areceived BCG signal.

FIG. 3 shows a block diagram of a system and approach 300 for detectingcardiac function and mitigating the effects of vibration in the localenvironment, according to another example embodiment of the presentinvention. The system 300 includes a BCG sensor 310 having a scale-typesensor as discussed above, a vibration sensor 320 and an adaptive noisecancellation circuit 330. A BCG characteristic detected via the sensorBCG 310 is processed with a secondary characteristic detected at thevibration sensor 320, representing vibration present in surroundingstructures. In some implementations, the sensor 320 may be placed todetect vibrations present in the BCG sensor 310 used to capture the BCGsignal. As such, this secondary vibration is detected using a sensorthat may be the same sensor as that used to detect the BCGcharacteristic, or a different sensor. By way of example, FIG. 3 shows ageophone sensor placed in close proximity to the BCG sensor/scale 310used to detect BCG characteristics. The detected vibration is passed tothe adaptive noise cancellation circuit 330 and used to process the BCGsignal (also provided to the circuit 330). The system output from theadaptive noise cancellation circuit 330 thus includes a BCG analysissignal with mitigated (or removed) vibration-based noise. For example,interference can be reduced in measured weight signals that occur due tothese vibrations (e.g., as part of an adaptive noise cancellationprocess).

FIG. 4 shows a circuit 400 for acquiring BCG signals from a commercialweighing scale, consistent with another example embodiment of thepresent invention. The circuit 400 is amenable to BCG acquisition from aweighing scale. The strain gauges within a commercial scale, such as anOmron HBF-500 scale, are arranged in a Wheatstone bridge configuration.The bridge is excited by a dc voltage of +/−9V, and the differentialvoltage across the bridge is amplified by an instrumentation amplifier(the LT1167) which is dc-blocked using integrative feedback (LT1014C).The output from this dc-blocked instrumentation amplifier stage is thenband-pass filtered and further amplified. The circuit gain is 90 dB,with a bandwidth sufficient for high-resolution BCG acquisition.

The specific circuit depicted by FIG. 4 is exemplary of a number ofdifferent implementations that can be used to provide similarfunctionality. As with other aspects of the present invention, thevarious functionalities can be implemented using combinations of generalpurpose computers configured by specialized software, programmable logicdevices, discrete circuits/logic and combinations thereof.

The following description references various Appendices submittedherewith, in connection with various example embodiments. Each of theseAppendices is fully incorporated herein by reference.

Referring to Appendix A (Adaptive Cancellation of Floor Vibrations),aspects of the present invention are directed to adaptive cancellationof floor vibrations, as applicable to one or more of the followingexemplary embodiments:

-   -   1. An auxiliary sensor, such as a geophone or accelerometer, is        placed on the floor in close proximity to a BCG sensor, such        that it detects the vibrations of the floor.    -   2. An adaptive noise canceller is configured, with a BCG signal        as a primary input, and a secondary signal as a reference input,        to generate a system output that includes a filtered BCG signal        with the vibration artifacts removed.    -   3. An adaptive canceller circuit is trained or otherwise        configured, before BCG measurements are taken from a person, by        placing weights on a scale that is used to capture a BCG signal.        Using this approach, a BCG component can be set to and any        measured signal is representative of floor vibrations. The        measured signal is thus used to identify floor vibrations and is        used in subsequent analysis. In some implementations, filter        coefficients are set based upon this measured vibration signal,        and is used to achieve a best least squares solution. The        coefficients can then be stored as initial conditions for an        adaptive filter used in ensuing BCG measurements, which can        facilitate real-time adaptive cancellation of floor vibrations        without a long delay at the beginning of the recording for the        system to converge.    -   4. BCG signals are captured from a number of subjects in a        transport vehicle, clinic, or similar setting, using a single        auxiliary sensor as a floor vibration noise reference for all        BCG sensors. In one example involving a vehicle such as a bus,        each seat is equipped with a BCG sensor and a single sensor        (e.g., geophone or accelerometer) is placed on the floor, acting        as a noise reference for all of the BCG measurements. Each BCG        sensor uses an adaptive canceller, with an adaptive filter using        a transfer function relating to the noise reference pick-up of        floor vibrations to the pick-up of each individual BCG sensor.

Referring to Appendix B (IEEE EMBS 2009 Conference Paper), aspects ofthe present invention are directed to BCG signal estimation and toCardiac Contractility Assessment Using BCG, as applicable to one or moreof the following exemplary embodiments:

-   -   1. A BCG “pulse response” is defined as a BCG characteristic for        each subject that may persist for a longer time period than a        single heartbeat. This pulse response may, for example, include        a mechanical response of the arteries and body to the pulse of        blood ejected by the heart; these mechanical structures may        continue oscillating long after this initial pulse of blood,        causing the average BCG response to be longer in duration than a        single heartbeat. In this context, the pulse response is used in        characterizing aspects of the subject from which the BCG        response is captured.    -   2. ECG R-wave timing is used as a timing reference to compute a        “short-window” ensemble average BCG. This short-window average        is then used to estimate the amplitude of each BCG heartbeat for        the entire recording. BCG heartbeats are then re-segmented using        an ECG timing reference with a “long-window” process. These        long-window beats are then averaged after subtracting        surrounding beats from each BCG heartbeat, yielding an        interference-cancelled long-window BCG pulse response.    -   3. Damping of the mechanical afterwaves of a BCG pulse response        are used to characterize arterial compliance (e.g., where less        compliant arteries provide less damping and can be characterized        as such).    -   4. The interval between the ECG R-wave and the BCG J-wave (R-J        interval) is inversely correlated to changes in cardiac        contractility. The R-J interval is used to characterize the        contractility, in which a higher contractility leads to a lower        R-J interval, and vice versa.    -   5. The signal-to-noise ratio (SNR) of each heartbeat is detected        using normalized ensemble correlation and, in some        implementations, R-J intervals are disregarded for heartbeats        with relatively lower SNR.    -   6. The time interval between the pre-ejection period (PEP) and        the R-J interval for each subject is used to characterize        arterial compliance. Less compliant arteries are detected        identified via shorter propagation delay between the ejection of        blood at the heart and a mechanical wave detected at the feet of        a subject (e.g., as akin to a rigid pipe propagating an acoustic        wave faster than a compliant, soft pipe).

Referring to Appendix C (Foot EMG), aspects of the present invention aredirected to using a foot-based EMG (Electromyogram) signal, asapplicable to one or more of the following exemplary embodiments:

-   -   1. Electromyogram (EMG) measurements taken from the feet of a        subject are used to detect (or flag) motion artifacts in the BCG        signal of a standing subject. These artifacts are        correspondingly used in processing or otherwise presenting a BCG        signal.    -   2. EMG measurements are recorded from footpad electrodes (e.g.,        as used for bioimpedance-based body-fat measurements) of        commercial weighing scales, and correspondingly used for        generating a BCG signal as described herein.    -   3. The moving root-mean-square (rms) power of the EMG signal,        from the feet, is used to determine whether the motion of the        subject is excessive. For instance, detecting a relatively        higher rms power of an EMG can be used as an indication of an        increase in motion, and accordingly used in considering any        measurements (e.g., to throw out or otherwise diminish the        application of a particular signal).    -   4. Time delays between an EMG signal and BCG noise are corrected        to correlate nose for effective noise detection.    -   5. The rms power of the EMG during each heartbeat is used for        generating a result using maximum likelihood weighted averaging        algorithms, in which a weighted ensemble averaged BCG beat is        computed by weighting each heartbeat in the ensemble average        higher for lower rms EMG power and lower for higher rms EMG        power.

Referring to Appendix D (Robust BCG Acquisition for Home Monitoring),aspects of the present invention are directed to BCG acquisition athome, as applicable to one or more of the following exemplaryembodiments:

-   -   1. A BCG signal is used in conjunction with bodyweight        measurements on a scale that is also used for monitoring the        health of heart failure patients at home. The BCG signal is used        to provide a measure of changes in perfusion by estimating        changes in cardiac output; bodyweight measurements can (e.g.,        simultaneously) provide an estimate of congestion by evaluating        weight change due to fluid retention. Both of these measurements        can be combined to provide a desirable assessment of a person's        cardiac health, since subjects can have congestion without        perfusion or perfusion without congestion.    -   2. Photoplethysmograph and ECG signals are used for averaging or        filtering a BCG signal, such as obtained herein.    -   3. Beat-by-beat BCG amplitude (J-wave) is used to characterize        the stroke volume for a particular beat to which the wave        applies.    -   4. The ECG R-wave timing is used as a timing reference to        compute a “short-window” ensemble average BCG. This short-window        average is then used to estimate the amplitude of each BCG        heartbeat for the entire recording. BCG heartbeats are then        re-segmented using the ECG timing reference, with a        “long-window” process. These long-window beats are then averaged        after subtracting surrounding beats from each BCG heartbeat,        yielding an interference cancelled long-window BCG pulse        response.

Referring to Appendix E (Non-invasive cardiac output trending usingBCG), aspects of the present invention are directed to non-invasivedetection of cardiac output, as applicable to one or more of thefollowing exemplary embodiments:

-   -   1. Changes in the BCG rms power are detected and correlated to        changes in cardiac output.    -   2. A single measurement is taken using a gold standard clinical        technique (e.g., Doppler ultrasound) to calibrate the rms power        of a subject to the subject's cardiac output. This baseline        value is stored in (or accessible to) the device taking BCG        measurements, and the baseline value is used to convert        subsequent measurements to absolute measurements of cardiac        output.    -   3. A circuit such as shown in FIG. 2 of Appendix E is used for        BCG acquisition, in connection with various example embodiments.    -   4. BCG measurements taken regularly over time and compared to a        baseline measurement, such as discussed above, and used to        identify therapeutic decisions such as diuretic dosage and        others.    -   5. BCG measurements are taken before and after exercise, and        features of the signal (such as rms power) are compared to        diagnose diseases and/or conditions. For example, a heart        failure patient may not see an increase in rms BCG power after        exercise, whereas a healthy person certainly would; detecting        rms BCG power can be used to identify relative heart conditions.

Referring to Appendix F (Valsalva Paper), aspects of the presentinvention are directed to using a Valsalva maneuver, as applicable toone or more of the following exemplary embodiments:

-   -   1. BCG measurements are taken during a Valsalva maneuver to        elicit various expected reflexes from the cardiovascular system.        A response to the Valsalva maneuver can be used to diagnose        diseases or conditions. For example, a patient with prior        myocardial infarction may not see increased BCG amplitude after        releasing the strain, whereas a healthy subject certainly does.        BCG amplitude can thus be monitored and used to identify such        conditions.    -   2. For cases when a simultaneous ECG is unavailable, a BCG        J-wave rise time is used as an indication of changes in cardiac        contractility.    -   3. A frequency domain analysis of a BCG signal is performed and        used to provide indications of the state of cardiac        contractility, by examining the ratio of high-frequency to        low-frequency power in the power spectral density of the BCG.

Referring to Appendix G (Two Electrode Biosignal Amplifier for ECGMeasurement), aspects of the present invention are directed to measuringan ECG, as applicable to one or more of the following exemplaryembodiments:

-   -   1. An ECG is measured in “current-mode” using a transimpedance        amplifier front-end, which leads to a low differential input        impedance, and mitigates microphonic cable noise that can occur        due to the movement of the cables during acquisition.    -   2. Current feedback is delivered to an input terminal using a        non-inverting integrator sensing low-frequency variations in an        output signal, and used to stabilize common-mode voltage at the        input and prevent amplifier saturation and other undesirable,        commonly-encountered problems in two-electrode (as opposed to        typical three-electrode) ECG recordings.    -   3. A micro-power op-amp is used with the bandwidth boosted by a        composite amplifier design, facilitating desirable current        consumption (e.g., about 3.9 micro-Amps), such that a battery        could operate the device continuously for years.    -   4. A lead-capacitor is used in the first stage of a composite        amplifier to set a second-order sharper roll-off in the overall        closed-loop response of the circuit, facilitating a greater        degree of attenuation at the Nyquist frequency for sampling in        analog-to-digital conversion of the signal.    -   5. A resistor is placed at a non-inverting terminal of an input        op-amp, connecting this terminal to ground, matching the        common-mode input impedances at the two input terminals. This        approach can be used, for example, to facilitate an optimized        (e.g., desirable) common-mode rejection ratio.    -   6. An ECG circuit is embedded in the handlebar electrodes of a        commercial weighing scale to provide an R-wave timing reference        for BCG signal averaging.    -   7. An ECG circuit is used for acquiring other biomedical        signals, such as electroencephalogram (EEG) signals from the        scalp.

The various embodiments described above are provided by way ofillustration, and should not necessarily be construed to limit theinvention. Based on the above discussion, those skilled in the art willreadily recognize that various modifications and changes may be made tothe present invention without strictly following the exemplaryembodiments and applications illustrated and described herein. Forexample, algorithms, calibration, and verification methods developed forthis system can be used for any BCG measurement system including bedsand tables. Other scale configurations, such as a seated or proneconfiguration, with the scale held vertically or at other relativeangles. Custom strain gauges can be used in lieu of a scale interfacedto the similar electronics as discussed herein. Several alternativeelectronics configurations are used for various embodiments, some ofwhich may include lock-in based circuits. Multiple scales can be used tomitigate or eliminate noise, such as by placing a scale can be placedunder each leg of a chair-based circuit, and by constructing a largerbridge circuit. A number of exemplary and experimental implementationsare discussed in detail in the appendices attached in theabove-referenced provisional application, which are fully incorporatedherein. The teachings of this disclosure include those teachings foundin the appendices (A-G) for much of the above-noted discussion ofexample embodiments, and the various teachings can be implemented eitheralone or in combination with one another. The skilled artisan wouldappreciate the contemplated context of the teachings found in theappendices, e.g., in light of overlapping technical discussion. Theseand other modifications and changes do not depart from the true spiritand scope of the present invention.

1. A system for providing BCG (ballistocardiogram) data from a user, thesystem comprising: a BCG capture device including a heart-characteristicsensor configured and arranged to capture, from the user, a BCG signalindicative of at least one of physical movement and mechanical output ofthe user's heart; a secondary sensor configured and arranged to detectan indication of at least one of noise present in the BCG signal and aphysiologic characteristic of the user, and to provide an outputcharacterizing the detected indication; and a processor circuit that isconfigured to use the secondary sensor output to process the capturedBCG signal and to generate an output BCG signal indicative of acondition of the user's heart.
 2. The system of claim 1, wherein the BCGcapture device includes a weighing scale, and the secondary sensorincludes an electrocardiogram (ECG) sensor configured and arranged todetect an ECG signal from the user, the detected signal being indicativeof a physiologic characteristic of the user, and to provide an outputcharacterizing the physiologic characteristic to the processor circuit.3. The system of claim 1, wherein the secondary sensor includes anelectrocardiogram (ECG) sensor configured and arranged to detect an ECGsignal from the user, the detected signal being useful forcharacterizing a physiologic characteristic of the user, and to providean output characterizing the physiologic characteristic to the processorcircuit, and the processor circuit is configured with an algorithm forprocessing the captured BCG signal to generate the output BCG signal,using the detected ECG signal as an input to the algorithm.
 4. Thesystem of claim 1, wherein the secondary sensor includes anelectrocardiogram (ECG) sensor configured and arranged to detect an ECGsignal from the user and to provide an output characterizing thedetected ECG signal, and the processor circuit is configured to use thesecondary sensor output to process the captured BCG signal by processingthe BCG signal by filtering and averaging the BCG signal to generate theoutput BCG signal.
 5. The system of claim 1, wherein the secondarysensor includes an electrocardiogram (ECG) sensor configured andarranged to detect an ECG signal from the user and to provide an outputcharacterizing the detected ECG signal, and the processor circuit isconfigured to generate the output BCG signal by generating at least oneof an ensemble-average and a triggered-moving average of the detectedBCG signal using the detected ECG signal.
 6. The system of claim 1,wherein the secondary sensor includes a photoplethysmography sensorconfigured and arranged to detect a photoplethysmography signal from theuser and to provide an output characterizing the detectedphotoplethysmography signal, and the processor circuit is configured togenerate the output BCG signal by generating an ensemble-average, ortriggered moving average, of the detected BCG signal using the detectedphotoplethysmography signal.
 7. The system of claim 1, wherein thesecondary sensor includes a noise sensor configured to detect noise inthe user's environment and a heart characteristic sensor configured todetect a physiologic characteristic of the user's heart, and theprocessor circuit is configured to generate the output BCG signal byusing the detected noise to mitigate noise in the captured BCG and usingthe detected heart characteristic to modify the captured BCG signal. 8.The system of claim 1, wherein the BCG capture device includes aweighing scale including the heart-characteristic sensor, and theprocessor circuit is configured to process the captured BCG signal,using previously-obtained data including cardiac output data for theuser and both rms power and an ensemble-average amplitude of the BCGsignal, to generate an output BCG signal indicative of the cardiacoutput of the user's heart.
 9. The system of claim 1, wherein the BCGcapture device includes a weighing scale including theheart-characteristic sensor, the processor circuit is configured toreceive cardiac data indicative of at least one of cardiac output andstroke volume obtained from the user, and process the captured BCGsignal, using the received cardiac data and both rms power and anensemble-average amplitude of the BCG signal, to generate an output BCGsignal indicative of the cardiac output of the user's heart.
 10. Thesystem of claim 1, wherein the BCG capture device includes a weighingscale including the heart-characteristic sensor, and the processorcircuit is configured to compare the captured BCG signal to a baselinevalue, which is based upon cardiac output data for the user and both rmspower and an ensemble-average amplitude of the BCG signal, to generatean output BCG signal indicative of the cardiac output and stroke volumeof the user.
 11. The system of claim 1, where the secondary sensor is anelectrocardiogram (ECG) sensor configured and arranged to detect an ECGsignal from the user and to provide an output characterizing thedetected ECG signal, and the processor circuit is configured to usepacing artifacts in the output from the ECG sensor to process thecaptured BCG signal by triggered averaging, ensemble averaging, orotherwise filtering the BCG signal.
 12. The system of claim 1, whereinthe secondary sensor includes an implantable cardiac resynchronizationtherapy (CRT) device that provides an output having respective left andright ventricular pacing pulse timing components, and the processor isconfigured to use the CRT output to process a left ventricular componentof the BCG signal using the left ventricular pacing pulse timingcomponent, and a right ventricular component of the BCG signal using theright ventricular pacing pulse timing component, for deconvolving theBCG signal via at least one of synchronous averaging or filtering, theleft and right components being statistically uncorrelated temporallydue to variation in inter-ventricular timing delay as is present inCRT).
 13. The system of claim 1, where the BCG capture device is anaccelerometer within an implantable device, and the processor circuitfilters the BCG signal using a pacing pulse timing output of theimplantable device to determine a diagnostic characteristic of the BCGsensor.
 14. The system of claim 1, wherein the BCG capture deviceincludes an implantable device having an accelerometer, and theprocessor circuit is configured to automatically set cardiac paceparameters for the user based on an output from the accelerometer. 15.The system of claim 1, wherein the processor circuit is configured togenerate an output to set cardiac resynchronization therapy (CRT) paceparameters for the user, using at least one of a maximum amplitude orminimum ECG R-wave to BCG J-wave interval of the output BCG signal. 16.The system of claim 1, wherein the processor circuit is configured togenerate an output BCG signal indicative of BCG signal variations due tochanges in cardiac resynchronization therapy (CRT) parameters used by animplantable CRT device to regulate the user's heart, to provide anindication of cardiac responsiveness to CRT parameters.
 17. The systemof claim 1, wherein the BCG capture device is integrated with abodyweight sensing scale, the secondary sensor is integrated withhandlebar electrodes coupled to the scale, the electrodes and secondarysensor being configured to detect at least one of an electrocardiogram(ECG) or photoplethysmography characteristic of the user, and theprocessor circuit is configured to generate output BCG signals over timeto provide an indication of at least one of cardiac output and strokevolume for determining a treatment need for the user.
 18. The system ofclaim 17, wherein the scale includes a memory that stores baseline dataincluding at least one of rms BCG data and ensemble average BCGamplitude data, and the processor circuit is configured to generateoutput BCG signals based upon the stored baseline data.
 19. The systemof claim 18, wherein the processor circuit is configured to use recordedchanges in the user's bodyweight over time for determining the treatmentneed for the user.
 20. The system of claim 1, wherein the BCG capturedevice is integrated with a bodyweight sensing scale, the secondarysensor is integrated with a detachable component connected to the scaleand is configured to detect a photoplethysmograph characteristic of theuser, and the processor circuit is configured to generate output BCGsignals over time to provide an indication of at least one of cardiacoutput and stroke volume for determining a treatment need for the user.21. The system of claim 20, wherein the scale includes a memory thatstores baseline data including at least one of rms BCG data and ensembleaverage BCG amplitude data, and the processor circuit is configured togenerate output BCG signals based upon the stored baseline data.
 22. Thesystem of claim 20, wherein the processor circuit is configured togenerate an output characterizing a treatment condition for the userbased upon recorded changes in the user's bodyweight and the output BCGsignals that are generated over a time period.
 23. The system of claim1, wherein the processor circuit is configured to generate an output BCGsignal indicative of BCG rms power to provide an indication of cardiacfitness.
 24. The system of claim 1, wherein the BCG capture device isintegrated with a bodyweight sensing scale, the secondary sensor isconfigured to detect at least one of an electrocardiogram (ECG) andphotoplethysmography characteristic of the user, and the processorcircuit is configured to generate output BCG signals over time toprovide an indication of at least one of cardiac output and strokevolume for setting a cardiac pacing parameter for the user, using thedetected characteristic from the secondary sensor as an input for atleast one of filtering and averaging the output BCG signals.
 25. Thesystem of claim 1, wherein the secondary sensor is configured togenerate an ECG signal using characteristics detected from the user, andthe processor is configured to modify coefficients of a transformfunction used to generate the output BCG signal, based upon the ECGsignal.
 26. The system of claim 1, wherein the secondary sensor includesan electromyogram (EMG) sensor that is configured to detect motion ofthe user standing on the BCG capture device.
 27. The system of claim 1,wherein the secondary sensor includes a foot-to-foot ECG sensor.
 28. Thesystem of claim 1, wherein the secondary sensor includes an opticalplethysmograph.
 29. The system of claim 1, wherein the secondary sensorincludes an accelerometer arranged to be affixed to the waist or chestof the user.
 30. The system of claim 1, wherein the secondary sensorincludes a geophone configured to detect vibrations to which the BCGcapture device is exposed.
 31. The system of claim 1, wherein thesecondary sensor includes a foot-to-foot impedance cardiogram sensor.32. The system of claim 1, wherein the processor circuit is configuredto use detected noise characteristics from the secondary sensor tomitigate noise via heart rate-triggered ensemble averaging.
 33. Thesystem of claim 1, wherein the processor circuit is configured to usethe detected noise characteristics for adaptive noise cancellation inthe BCG signal, using a signal from the secondary sensor as a noisereference.
 34. The system of claim 1, wherein the processor circuit isconfigured to use the detected noise characteristics to adaptivelyenhance the BCG signal.
 35. The system of claim 1, wherein the secondarysensor is configured to detect noise that includes interference presentin the BCG signal.
 36. The system of claim 1, wherein the processor isconfigured to generate the output BCG signal for characterizing theresponsiveness of the user's heart to parametric adjustments made tostimulus applied to the heart via cardiac resynchronization therapy(CRT).
 37. The system of claim 1, wherein the processor is configured togenerate the output BCG signal indicative of the cardiac output of theuser's heart.
 38. The system of claim 1, wherein the processor isconfigured to generate the output BCG signal indicative of the leftventricular mass (LVM) of the user's heart.
 39. The system of claim 1,wherein the secondary sensor includes a non-contact (RF) sensorconfigured to measure at least one of heart or chest-wall movement.